1. Field of the Invention
The present invention relates to actuated safety relief valves. More specifically, the present invention relates to actuated relief valves that use a sacrificial buckling pin as a trigger for an actuator and to pilot valve safety relief system.
2. Background of the Invention and Related Art
The use of sacrificial buckling pins is widely known in the valve industry. Through the selection and use of appropriate materials, buckling pins can be designed to fail with a high degree of reliability and accuracy upon the application of a predetermined axial compressive force. The maximum compressive load for a buckling pin is dependent on its slenderness, which is a function of the pin length and the diameter of its cross-section. For compressive axial failure, the ultimate compressive capacity of the pin is easily calculated using Euler's law relating the critical force to the modulus of elasticity of the material, the minimum moment of inertia of the cross section and the unsupported length of the pin. This principle does not apply if the bucking pin material is subject to an intervening mode of failure due to eccentric loading, material defects or yield stress limitations. The use of reasonably sized buckling pins is ideal for relief valve applications because of the simplicity of the device, the ease of replacement, low manufacturing, maintenance and upkeep costs, and the elimination of complicated electronic, pneumatic or spring mechanisms common in many existing valve actuators.
The primary objective of safety relief valve designers is to obtain the maximum fluid flow capacity in order to relieve excessive system pressure. High flow capacities require a large orifice at the valve seat, especially in low pressure applications. In existing safety relief valves that use sacrificial buckling pins, the size of the valve orifice is directly determined by the physical displacement of the "active" or moving end of the buckling pin upon buckling failure. In other words, the physical collapse or "stroke" of the buckling pin directly determines the size of the orifice opened for relief flow. Consequently, the conventional use of buckling pins limits the relief capacity of the valve, requiring either redundant valves or excessively large valves in order to obtain the desired flow capacity.
The ultimate failure load of a buckling pin is more predictable if the length and size of the pin fall within a range of favorable slenderness ratios. With existing buckling pin design large valves or high pressure valves require a very long or large buckling pins, often resulting in a buckling pin that is of an awkward length or size for reliable prediction of buckling pin failure. Unfavorable slenderness ratios and intervening failure modes related to material yield stress, material defects or eccentric loading causes problems with buckling pin design and selection.
In safety relief valves, it is important to create a large orifice at the valve seat to quickly relieve system pressure by rapid removal of gas or liquid from the system. The required size of the orifice necessarily depends on the available pressure differential across the valve, the desired flow rate and fluid properties. Generally, the smaller the stroke of the valve, the smaller the orifice made available for relief flow, the larger the valve must be in order to achieve its purpose.
FIGS. 1(a) and 1(b) show a prior art relief valve in closed and open positions, respectively. The prior art valve uses a buckling pin as a direct and stand alone actuator to oppose the full process pressure applied against a relief valve. In this relief valve, the stroke is determined by the difference in the original and failed buckling pin lengths. High flow capacities in existing buckling pin actuator valves are obtained only by increasing the length of the buckling pin in order to increase displacement of the valve flapper, piston, plug or other actuated component. Longer collapse displacements required by large valves require longer, larger and more expensive buckling pins. Reliability and accuracy of predicted buckling pin failure loads are lost due to unfavorable slenderness ratios and the increasing intervention of other pin failure modes. Intervening failures related to material defects, material yield stress limitations and manufacturing irregularities may determine the ultimate load of the buckling pin.
Other existing valves may use mechanical linkages to mechanically reduce the force applied to the buckling pin in order to keep buckling pin slenderness ratios in the favorable range. Mechanical linkages using the lever arm principle provide a means of scaling the force applied to the linkage down to a manageable level. Inaccuracies and poor reliability result from wear or friction losses introduced by movable mechanical joints, all of which are magnified by the scaled mechanical advantage gained in the linkage, and result in an overall loss of valve accuracy and reliability. Therefore, there is a need for a simplified buckling pin actuated relief valve that avoids the problems of limited displacement, oversized buckling pins and inaccurate mechanical linkages. There is also a need for a durable, low maintenance buckling pin actuated safety relief valve that is easily reset and returned to service and inexpensive to manufacture and maintain.